A conventional pneumatic brake system for a large heavy vehicle, such as a truck, a bus, a trailer, etc., includes an air compressor, an air reservoir carrying pressurized air from the air compressor, a conduct line connected with the air reservoir and a relay valve, a plurality of brake cylinders connected with the relay valve, a first brake line connected with the air reservoir and a brake pedal, and a second brake line connected with the brake pedal and the relay valve.
In a normal driving situation, pressurized air is sent from, the air reservoir to the relay valve and the brake pedal through the conduct line and the first brake line, respectively, but does not flow through the relay valve so as not to actuate the brake cylinders for applying a braking action.
When a driver depresses the brake pedal to cause pressurized air to pass through the second brake line, the pressurized air urges the relay valve so as to open the relay valve, whereby the pressurized air with a high pressure flows to the brake cylinders for applying a braking action.
However, in a long vehicle, such as a bus, the second brake line must be relatively long and air pressure therein has to be sufficient to activate the relay valve, which leads to a long response time and results in a time delay of the braking action.